Biosensors are devices that identify chemical or biological entities with high selectivity on the basis of molecular recognition. Biosensor devices integrate the assay substrate and sensor element into a single device.
The advantage of most bio-sensing devices lies in the extremely high specificity of interactions of biological molecules. Thus, biosensors have been based on the specific interactions between enzymes and their substrates, the recognition between antibodies and antigens, accessibility of specific target molecules (ligands) to their receptors, or the high affinity of nucleic acids strands to their complementary sequences. In all of these, the focus is on the specificity endowed by the unique recognition of two molecules.
A completely different analytical approach is allowed when the biological entity in question is not a molecule but rather a live, intact cell [1, 2, and 3]. The intact, living microbial cells have been genetically engineered to produce a measurable signal in response to a specific chemical agent or physical change in their environment. This triggers very complex series of reactions that can exist only in an intact, functioning cell. Thus, global parameters such as toxicity or genotoxicity can be assayed by using whole cells. Molecular recognition or chemical analysis can rarely provide this type of information. Among whole-cell biosensors, the most promising ones to date are those based on single cell organisms such as bacteria or yeast [2, 4, and 5].
Microbial biosensors are based on the ability of microorganisms to continuously monitor their microenvironment and respond to local environmental changes by expressing specific expression products. In recent years genetic engineering methods have been developed to create types of bacteria that respond to minute quantities of specific chemical or biological substances. The response of the bacteria may be manifested in different ways which include the generation of either luminescent or florescent molecules, or by the production of a minute electrical charge (for example due to production of ionic channels). This response can be detected by a sensitive measurement system that generates an electronic signal the analysis of which yields an indication of the presence of the pursued substance.
As illustrated in FIG. 1, biosensing microorganisms contain two essential genetic elements, a transcription regulatory element (promoter and/or enhancer sequence) and a reporter gene coding for an expressible product. The promoter element is turned on when the target agent is present in the cell's environment. Turning on the promoter causes the reporter gene to be transcribed. Activation of the reporter gene leads to production of reporter proteins that generate some type of a detectable signal either directly (by florescence or luminescence) or by working enzymatically on substrates. Therefore, the presence of a signal indicates that the biosensor has sensed a particular target agent in its environment. For several types of biosensors using for example Green fluorescent protein (GFP) as exemplified in FIG. 1, or Uroporphyrinogen (Urogen) III Methyltransferase (UMT) as reporter gene, the signal must be activated by an external light source.